Iron turnover in the skin

Regulation of Epidermal Ferritin Expression Influences Systemic Iron Homeostasis

Absorption of dietary iron is largely regulated by the liver hormone hepcidin, which is released under conditions of iron overload and inflammation. Although hepcidin-dependent regulation of iron uptake and circulation is well-characterized, recent studies have suggested that the skin may play an important role in iron homeostasis, including transferrin receptor-mediated epidermal iron uptake and direct hepcidin production by keratinocytes. In this study, we characterized direct keratinocyte responses to conditions of high and low iron. We observed potent iron storage capacity by keratinocytes in vitro and in vivo and the effects of iron on epidermal differentiation and gene expression associated with inflammation and barrier function. In mice, systemic iron was observed to be coupled to epidermal iron content. Furthermore, topical inflammation, as opposed to systemic inflammation, resulted in a primary iron-deficiency phenotype associated with low liver hepcidin. These studies suggest a role for keratinocytes and epidermal iron storage as regulators of iron homeostasis with direct contribution by the cutaneous inflammatory state.

Iron Metabolism of the Skin: Recycling versus Release

The skin protects the body against exogenous stressors. Its function is partially achieved by the permanent regeneration of the epidermis, which requires high metabolic activity and the shedding of superficial cells, leading to the loss of metabolites. Iron is involved in a plethora of important epidermal processes, including cellular respiration and detoxification of xenobiotics. Likewise, microorganisms on the surface of the skin depend on iron, which is supplied by the turnover of epithelial cells. Here, we review the metabolism of iron in the skin with a particular focus on the fate of iron in epidermal keratinocytes. The iron metabolism of the epidermis is controlled by genes that are differentially expressed in the inner and outer layers of the epidermis, establishing a system that supports the recycling of iron and counteracts the release of iron from the skin surface. Heme oxygenase-1 (HMOX1), ferroportin (SLC40A1) and hephaestin-like 1 (HEPHL1) are constitutively expressed in terminally differentiated keratinocytes and allow the recycling of iron from heme prior to the cornification of keratinocytes. We discuss the evidence for changes in the epidermal iron metabolism in diseases and explore promising topics of future studies of iron-dependent processes in the skin.

Epidermal iron metabolism for iron salvage

BACKGROUND

The epidermis shows a reverse iron gradient from the basal layer to the stratum corneum and consequently, little epidermal intracellular iron is lost by desquamation.

OBJECTIVE

To clarify the underlying mechanism of iron salvage.

METHODS

We first used immunohistochemistry and mRNA quantification to demonstrate the distinctive expression pattern of iron metabolism molecules. The obtained results were confirmed using normal human epidermal keratinocytes (NHEKs) during in vitro differentiation. We next examined the effects of reducing ferroportin expression in vitro by ferroportin-specific siRNAs or hepcidin on the intracellular iron content of cultured NHEKs. Finally, we compared epidermal and systemic iron metabolism between Fpn^Epi-KO mice and control mice.

RESULTS

The results of both mRNA and protein expression analysis showed that most molecules participating in iron import and storage were expressed in the lower epidermis, while those involved in iron release from heme or iron transport were expressed in the upper epidermis. Consistent with their expression, keratinocyte differentiation reduced intracellular iron content. We next demonstrated that reducing ferroportin expression in vitro by ferroportin-specific siRNAs or hepcidin significantly increased the intracellular iron content. Finally, we showed that the iron content of the epidermis and squames was significantly greater in Fpn^Epi-KO mice than in control mice, and that Fpn^Epi-KO exhibited a more rapid decrease in blood hemoglobin concentration than control mice on a low iron diet.

CONCLUSION

These studies demonstrated that the epidermis is equipped with a machinery by which intracellular iron in differentiated keratinocytes is excreted to the extracellular space before reaching the stratum corneum.

Iron deficiency and iron overload

An up to date review of our knowledge of human iron metabolism is given including problems of iron balance, internal transport, and intracellular mechanisms. Current knowledge of the iron proteins is summarized and this background is used in discussing the pathophysiology of iron deficiency and overload, together with the internal derangements such as sideroblastic anemia which form much of the clinical practice associated with disorders of iron metabolism. The therapeutic approach to these problems will be described.